This application is based on French Patent Application No. 01 01 452 filed Feb. 2, 2001, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. xc2xa7119.
1. Field of the Invention
The invention relates to a saturable optical absorber and its application to regenerating a wavelength division multiplexed signal.
2. Description of the Prior Art
A saturable optical absorber is an optical device consisting of a material adapted to absorb an optical signal of low luminous power but to be transparent at high powers. The material of a saturable absorber becomes increasingly transparent to a light beam as the power of the beam increases.
One example of the use of a saturable absorber is to attenuate the unwanted noise between pulses of a power-modulated optical signal.
When a saturable absorber receives a pulse of optical power it becomes transparent and allows the pulse to pass through it. On the other hand, the saturable absorber becomes absorbant and attenuates unwanted noise between the pulses.
In the case of wavelength division multiplexed optical signals, the signal on each channel is power modulated. Thus saturable absorbers can also be used to regenerate wavelength division multiplexed signals.
However, the material constituting saturable absorbers used until now to regenerate wavelength division multiplexed signals has imposed the use of a demultiplexer for spatially separating the optical components with different wavelengths forming the multiplexed signal. The regenerated wavelength division multiplexed signal is obtained by means of a multiplexer after passing these components through a saturable absorber medium.
A wavelength division demultiplexer is required because it is necessary to separate the channels of the signal to direct them to the absorber, not at a single point, as would be the case in the absence of demultiplexing, but at a plurality of clearly separated points, each point corresponding to a channel of given wavelength.
The signal on a channel of wavelength xcex1 consists of temporal pulses (corresponding to modulation of the power of the optical signal) representing high logic levels xe2x80x9c1xe2x80x9d and low logic levels xe2x80x9c0xe2x80x9d. Existing saturable absorber members consist of a material with uniform quantum boxes exhibiting homogeneous absorption throughout a range of wavelengths. FIG. 1B shows the absorption curve of a member of this kind. The dashed line curve SO corresponds to the absorption spectrum for a wave of low luminous power and the continuous line curve SA corresponds to the absorption spectrum for a wave of high luminous power. The figure shows clearly that the whole of the spectrum within the absorption band is modified.
At a given time the signals corresponding to some channels can have a high level xe2x80x9c1xe2x80x9d while the signals corresponding to other channels have a low level xe2x80x9c0xe2x80x9d. Accordingly, in the absence of demultiplexing, the unwanted noise present at low power levels on some channels is masked by the high levels on other channels. Thus the saturable absorber member cannot reduce noise at low power levels if high levels are present elsewhere.
Consequently, the saturable absorber member is ineffective for a wavelength division multiplexed signal in the absence of demultiplexer and remultiplexer devices.
The present invention proposes a simple solution that does not necessitate the addition of a multiplexer and a demultiplexer and is relatively inexpensive and more compact compared to the prior art solutions.
The present invention therefore provides a saturable optical absorber including an inhomogeneous absorption material including a plurality of sets of quantum boxes, wherein the sets are associated with respective different absorption wavelengths, the quantum boxes of a given set have dimensions chosen to obtain an absorption wavelength associated with the set, the quantum boxes are disposed in a plurality of stacked layers and the absorber is adapted to enable an incident optical wave to pass through the stacked layers.
Thanks to this feature, it is possible to regenerate a plurality of channels of a wavelength division multiplexed signal without having to demultiplex it. Furthermore, the fact that the optical wave can be processed by propagating through layers of quantum boxes, i.e. with no guiding medium, in the direction perpendicular to the planes of the layers, has the advantage that the device is insensitive to the states of polarization of the channels.
In a first embodiment, each of the layers includes quantum boxes of the same set, layers belonging to different sets are stacked and form an individual absorption structure. The absorber includes a stack comprising a plurality of individual absorption structures.
In a second embodiment, the stack comprises a plurality of layers each including quantum boxes of a plurality of sets.
To limit the height of the stack, and consequently its overall size, and also to limit the fabrication time of the component, the absorber includes two mirrors disposed on respective opposite sides of the stack and parallel to or slightly inclined to the stack.
If the mirrors are parallel to each other, the component obtained forms a resonant cavity inside which the input optical signal is reflected several times and passes through the layers of quantum boxes several times.
In one embodiment the saturable absorber material consists of a wetting layer of a binary material, such as indium arsenide (InAs), followed by a layer of quantum boxes, this alternation of wetting layers and layers of quantum boxes being repeated, the combination being deposited on a substrate, for example of gallium arsenide (GaAs).
The invention also provides an application of a saturable absorber having the above features to regenerating a wavelength division multiplexed signal, wherein the wavelengths associated with the sets are respective optical carrier wavelengths of the multiplexed signal.
Other features and advantages of the invention will become apparent on reading the following description, which is given with reference to the accompanying drawings.